1. Field of the Invention
This invention relates to voltage reference circuits, and more particularly to a voltage reference circuit which is characterized by an output voltage which varies with temperature in a predetermined manner, and which includes a compensation subcircuit to adjust the reference output in a manner complementary to its natural temperature variation, thereby reducing the reference's net temperature variation.
2. Description of the Prior Art
Voltage references are required to provide a substantially constant output voltage irrespective of changes in input voltage, output current or temperature. Such references are used in many design applications, such as digital-to-analog convertors, power supplies, cold junction thermistor compensation circuits, analog-to-digital convertors, panel meters, calibration standards, precision current sources and control set-point circuits.
Modern voltage references are generally based on either zener diodes or bandgap generated voltages. Zener devices characteristically exhibit high power dissipation and poor noise specifications. Bandgap voltage references, on the other hand, are designed to yield stable output voltages over temperature by summing a pair of voltages with negative and psoitive temperature coefficients. A voltage with a negative temperature coefficient is obtained from the base-emitter junction of a transistor, while a voltage with a positive temperature coefficient is obtained from the difference between the base-emitter voltages of two transistors operating with unequal current densities. When the differential voltage is amplified and added to the base-emitter voltage of the first transistor, a voltage level with a very low temperature coefficient results if the sum equals 1.23 volts. The 1.23 volt level is then amplified to provide stable output voltages of typically 5.0 and 10.0 volts.
Presently available bandgap voltage references are unfortunately not totally insensitive to temperature, and in some cases the temperature dependence reaches unacceptable levels. A prior art circuit which was developed in an attempt to balance out temperature induced voltage variations is illustrated in FIG. 1, which is a simplification of the circuit described in an article by G. McGlinchey, "A Monolithic 12b 3 us ADC", 1982 IEEE International Solid-State Circuits Conference Digest, page 296, FIG. 4. This circuit has a temperature compensation feature which noticeably reduces the temperature dependence of the output reference voltage. However, the circuit requires both a positive and a negative voltage supply, whereas stand alone voltage references normally require only a positive voltage supply. The user would normally have to provide the negative voltage supply, thus adding to the cost and complexity of the system. Furthermore, this prior art circuit has no convenient mechanism to compensate for processing variations, which effect the nature of its temperature dependence.
Referring to the details of the FIG. 1 circuit, a bandgap voltage reference circuit 2 is shown enclosed in dashed lines. The circuit includes an output amplifier 4, a resistor-transistor network 6 which provides positive and negative inputs to the amplifier, a positive voltage supply terminal 8 and an output impedance circuit consisting of resistors R1 and R2 connected in series between the output of amplifier 4 and ground. The junction between R1 and R2 serves as a bias point for transistors in the reference circuit.
The reference voltage at the output of amplifier 4 supplies power to a pair of current sources I1 and I2, which are connected to ground through diode-connected transistors T1 and T2, respectively. The magnitude of I1 is set at a constant value I.sub.c, typically 60 microamps. The magnitude of I2 is set equal to I1 times T/T.sub.0, where T is absolute temperature and T.sub.0 is reference temperature, typically 25.degree. C. The McGlinchey reference illustrates circuitry which may be used to establish I1 and I2. The bases of T1 and T2 provide differential inputs to a differential amplifier consisting of transistors T3 and T4, the emitters of which are connected together. A current source I3 is connected to a negative voltage supply terminal 10 and draws current through the differential amplifier transistors.
The collectors of differential amplifier transistors T3 and T4 are coupled together by means of a mirror circuit comprising transistors T5 and T6, the mirror circuit being supplied with power from the reference voltage output terminal. The current through T4 relative to T3 is established by the current through T2 relative to T1, which in turn varies with temperature in accordance with the relationship T/T.sub.0 between I2 and I1. When the temperature rises above T.sub.0, the current transmitted through T2 by I2 increases by an amount proportional to the temperature rise above T.sub.0. The greater bias on T4 increases the current flow through that transistor, which through the action of the differential amplifier produces a corresponding drop in the current through T3. The current through T5, which is connected in series with T3, will drop by the same amount as the current drop through T3, and this current drop is reflected by the mirror circuit as a similar drop through T6. The current through T6, which is connected in series with T4, will thus be less than the current through T4 by an amount equal to the combined current rise through T4 and the current drop through T3.
The difference between the T4 and T6 currents is supplied as an output corrective current I.sub.0 over line 12 from the junction of R1 and R2 in the voltage reference output impedance circuit. This current is delivered from the voltage reference output through R1, thus increasing both the voltage across R1 and the reference voltage at the output V.sub.0 of amplifier 4. In this manner a drop in the reference voltage resulting from a temperature rise is compensated by an increase in the compensation current delivered along line 12 to the output impedance circuit, which tends to compensate for the reference voltage swing.
The above compensation technique is illustrated in FIGS. 2 and 3. FIG. 2 illustrates the output reference voltage without temperature compensation. The voltage is at a desired reference value at temperature T.sub.0 at the lower end of its operating temperature range, and prgressively drops as the temperature increases. Its value has been found to be a function of (kT/q)ln(T/T.sub.0), where k is Boltzmann's constant and q is the electronic charge. The compensation current I.sub.0, illustrated in FIG. 3, begins at substantially zero at a temperature of T.sub.0, and progressively increases with increasing temperature. The circuit is designed so that the reference output voltage adjustment produced by I.sub.0 substantially balances out the reduction in the reference voltage caused by increasing temperature, resulting in a substantially constant output reference voltage (the slopes of the curves in both FIGS. 2 and 3 are exaggerated for purposes of illustation).
While the described compensation circuit considerably improves the temperature performance of the voltage reference circuit, as noted above it requires a negative voltage supply that otherwise would not be needed. In addition, it requires a matching of numerous circuit elements in order to conveniently adjust the circuit to compensate for processing variations in its manufacture.